Accelerometers are electromechanical devices that can measure acceleration forces due to motion or vibration. Accelerometers are used in a wide variety of applications, including seismic sensing, vibration sensing, inertial sensing and tilt sensing. Electromagnetic force feedback accelerometers with closed loop electronics use a very low resonance frequency sensor which has a high sensitivity. Such accelerometers may be used for measurement, control and navigation e.g. in civil aerospace applications. In accelerometers such as the Q-flex series available from Honeywell, the resonance frequency of the sensor is typically about 10 Hz, which decouples the sensor from its support, and gives very low bias errors e.g. less than 15 mg. However, the use of electromagnetic coils is difficult to implement in a micro electromechanical systems (MEMS) structure, so these devices are relatively large and expensive.
Capacitive accelerometers are typically manufactured from silicon and fully implemented as MEMS devices, using electrostatic forces for closed loop operation. These accelerometers are smaller, cheaper and more reliable than equivalent electromagnetic devices. MEMS capacitive sensors combined with advanced electronics are replacing traditional geophones or seismometers for different types of seismic sensing. A typical seismic sensor available from Colibrys provides a g range of 1-5 g, combined with low noise and high resolution. These devices are sensitive to the low amplitude signals produced by natural vibrations. However there remains a need for a high sensitivity accelerometer, with a micro-g bias stability, but also a ±30 g range making it suitable for navigation as well as seismometery.
A typical MEMS capacitive accelerometer comprises a proof mass movably mounted relative to a support and sealed so that a gaseous medium trapped inside the device provides squeeze damping for the proof mass when it moves in a sensing direction in response to an acceleration being applied. There is typically provided a set of fixed electrodes and a set of movable electrodes attached to the proof mass, with differential capacitance between the electrodes being measured so as to detect deflection of proof mass. WO 2004/076340 and WO 2005/083451 provide examples of capacitive accelerometers comprising a plurality of interdigitated fixed and moveable electrode fingers extending substantially perpendicular to the sensing direction of the MEMS device. A pulse width modulation (PWM) technique may be used to control the voltage waveforms supplied to the fixed electrodes. An in-phase PWM waveform is applied to a first set of fixed electrode fingers while an anti-phase PWM waveform is applied to a second set of fixed electrode fingers. In such a PWM regime the mark/space ratio varies with applied acceleration and provides a linear measure of acceleration. In WO 2005/083451 a servo adjusts the time difference of the mark/space ratio of the PWM drive signals so as to linearise the output of the accelerometer with input acceleration. Variations in the sensor system, such as temperature variations and/or mechanical variations in the construction of the MEMS device, can change the lateral gap between the interdigitated electrode fingers. This may be compensated by varying the mark/space ratio, to maintain the proof mass at a null position at all times. The voltage Vcrit at this operating point determines the maximum acceleration g that can be detected as it can be difficult to obtain a stable servo loop at a voltage significantly larger than Vcrit. The value of Vcrit varies as Ω2, so lowering the resonance frequency Ω of the proof mass can move operation of the accelerometer into the nano-g range by increasing the sensitivity, for example ±30 g.
A high performance accelerometer such as is described in WO 2005/083451 can cover a g range of ±50 g or ±75 g with an accuracy of +/−1 milli-g, which is adequate for certain military aerospace applications. In these devices the proof mass has a resonance frequency typically in the range of 1-3 kHz, which is relatively high, using approximately critical damping. The MEMS design limits the performance of the sensor to a noise figure of about 20 micro-g/root Hz. The damping is provided by air squeeze film damping with atmospheric pressure gas, such as argon or neon. For example, the Gemini accelerometer available from Silicon Sensing Systems Ltd. is a high performance dual-axis silicon MEMS accelerometer sensor having a dynamic range between ±0.85 g and ±96 g for different variants. The proof mass is formed by etching completely through the silicon, with typically 100 micron deep trenches into the silicon substrate. A limitation of the current MEMS design is the aspect ratio of the trenches defined by the depth over the gap, which is set to 20 for ease of manufacture. Thus for a substrate depth of 100 microns the minimum trench width is limited to 5 microns, which represents the lateral spacing between the interdigitated electrode fingers. This spacing limits the squeeze damping to a damping factor of about 1 for representative designs, which makes it difficult for the proof mass to have a low resonance frequency and low noise.
It would be desirable to reduce the resonance frequency Ω of a MEMS capacitive accelerometer so as to improve the bias performance, as the open loop gain is proportional to 1/Ω2. If it is possible to reduce the resonance frequency, this increases the open loop sensitivity of the MEMS sensor to acceleration in terms of a capacitance change per g reducing noise and improving bias stability.